A time when something was difficult and you did well You should say: What it was How you did/completed it Why it was difficult And how you felt about doing it

If you're talking about something challenging that I have successfully conquered, then it has to be my undergraduate thesis. It was this long, 100-page thesis with a really complicated topic, and I had to finish it in a rush. Completing it was quite an achievement for me. It happened during my senior year of university [...]